The dye 1-ethyl-2-(3-(1-ethylnaphtho(1,2d)thiazolin-2-ylidene)-2-methyl-propenyl)naphtho (1,2d)thiazolium bromide is a metachromatic dye of unusual sensitivity for differentiating various classes of macromolecules by color. Methods for staining proteins, phosphoproteins, sialoglycoproteins, glycosaminoglycans and nucleic acids differentially in tissue sections and on polyacrylamide gels following electrophoresis were developed. Polar lipids could also be distinguished from the other classes of macromolecules in cryostat sections. These methods were applied to the study of milk and breast tissue proteins. The sensitivity of the method permitted the identification of phosphoproteins in less than 1 micron 1 of milk. In addition, the techniques enabled the determination of which four molecular weight species of caseins contained sialic acid. A histochemical study of normal and neoplastic human breast tissue is in progress. It is planned to apply the staining procedures to cytologic and biopsy specimens to determine whether they have special utility for the detection of early neoplastic lesions.